# Help with my design: How can I add these voltages?

I have to design a circuit that simulates a voltage spike on a dc supply rail. The circuit has a button that, when pressed, generates a short pulse of controllable height (controlled by pot). The internal circuitry essentially has to sit a pulse train on top of a constant dc voltage.

The voltage values are somewhat strict; they are shown in the diagram. The pulse height is controlled anywhere from 0v to 4v and the dc offset is 5.28v. From the diagram, you can see that the output from the 555 and the dc offset voltage are buffered for isolation.

My problem is that with a single rail power supply, the op amps cannot do the summing because the output will be inverted.

Use a non-inverting summing amplifier instead. They have a few drawbacks but I can't see any of them affecting you.

Your last stage, from R7 and R6 should look like this:

simulate this circuit – Schematic created using CircuitLab

I haven't done any simulations or calculations, so make sure you do that before implementing.

• Do you think the output signal should be fine if I use a rail-to-rail opamp? 720mV between V(top) and power seems to be cutting it a little close. – Mode77 Aug 27 '19 at 21:37

A fairly simple approach is to run the 555 (and associated circuitry) with a VCC of 10V and a VEE of 5V. This would give you an output switching between about 5V and 10V. A 3-resistor divider could bring it in to the specified 5.28V and 9.28V before the op amp buffer.

• I like this approach, but I think I'm misunderstanding. When the 555 output is LOW, wouldn't the voltage be 5V referenced to ground? The dc offset has to be 5.28V. – Mode77 Aug 27 '19 at 21:23
• That's why the 3-resistor divider. One goes to +10V, one to +5V, and one to your 555 output. The exact values will be determined by the exact voltages produced by the 555. – Cristobol Polychronopolis Aug 28 '19 at 12:57